Isabella was the queen of the Spanish kingdom of Castile and Ferdinand was the king of the Spanish kingdom of Aragon. In the year 1469, they were married, forming a union between the 2 kingdoms, uniting Spain once Ferdinand became king in 1479. In 1492 they finished the Reconquista. In 1516 their son Charles I would become king of both kingdoms, while at the same time being the Holy Roman Emperor. In the same year that they finished the Reconquista, Ferdinand and Isabella accepted and funded Christopher Columbus to sail across the Atlantic to find India and enter a faster spice Trade than the overland silk road route.
Christopher Columbus was an Italian explorer who upon asking multiple kingdoms for the funds to sail west, was finally accepted by Spain in 1492. He (with the exception of the Norse/Vikings) made the permanent discovery of "The New World", with his discovery of the Bahamas, Cuba, and Hispaniola. He claimed these for Spain, and brought back many products such as gold and natives whom he dubbed "Indians" since he thought he had reached India. He gave Spain the lead, he started the colonial age, and he got Spain closest to the gold and silver of Central America.
The Treaty of Tordesillas was a pact signed in 1494 that split the new world between the kingdoms of Spain and Portugal. It was a papal sponsored treaty, so no other catholic monarchs were allowed to take those lands, however, England and the Dutch, who would become protestant or reformed didn't have to abide by that, and France who always had tensions with the Papacy chose not to follow it, and the Papacy was too weak to challenge them. The Spanish and Portuguese followed the treaty, which effectively gave Portugal all of Brazil, and everything else that could be gobbled up would go to Spain.
The Spanish, led by conquistador Hernan Cortez, landed in what was the Aztec Empire in modern Mexico in 1519. Cortez with a few hundred Spanish troops was able to take over the empire by making a puppet out of their ruler Moctezuma, and by playing the vassals of the Aztecs against them. The Spanish brought them many diseases such as small pox that the natives were not immune to, and the populations of the natives were drastically reduces, severely weakening them. The empire, and all of modern Mexico soon fell to the Spanish. This region, along with central America were filled with silver and gold deposits that greatly enriched the Spanish, and promoted greater European colonization. The Spanish conquest of the Aztecs led to Spanish domination of Central America until the 1800s.
The Inca Empire was in what is modern day Peru. They were a wealthy, powerful, and large empire. However, when the Spanish discovered them, they were in the midst of a civil war, and soon after were plagued with disease that wiped out much of their population and military. The Spanish led by Francisco Pizarro, took advantage of the political instability and disease and took over the Empire. Pizarro had landed in 1526, and the last Inca stronghold was conquered in 1572. This conquest led to Spanish domination of Western and Northern South America until the 1800's.
The encomienda system was used during the Spanish colonization of the Americas, whereby conquistadors were granted the towns of the native people they conquered. The Spanish theoretically taxed these native people and used them for labor in exchange for agreeing to provide safety through an established military and religious teachings. However, most if not all of the Spaniards used their influence and power to take more land from the natives, increase taxes, and ultimately force the natives into slavery, resulting in many deaths and rebellions, so much so that sometimes there were just no natives left, so the Spanish imported African Slaves. Because of the ruthlessness of the system, native populations fell, and African slaves were introduced.
The Spanish Vice-royalties was their form of governance over the colonial conquests. These are similar to that of ancient satrapies because the leaders were viceroys or governors that were independent in actions, but served the Spanish crown and were not hereditary. These also mark the general boundaries of the modern states that formed from the Spanish colonies. The very first one was "New Spain", formed in 1521 in modern day Mexico.
The Spanish created hundreds of Spanish missions, organized around churches. These served to appease the Papacy, spread Christianity, united the natives around the missions. They provided labor and developed the colonial towns and outposts. These missions formed the primary form of Spanish presence and power since few Spaniards actually went to the Americas to colonize.
Spanish colonialism in the 15th and 16th centuries saw their Empire spread to much of north and South America and parts of East Asia such as the Philippines. They gained immense wealth from trade and resources such as gold and silver deposits. The Spanish Empire became the richest and most powerful empire in its time. However, due to large inflation and many costly wars with its neighbors, their power was reduced and corruption grew. Additionally, because few Spaniards actually stayed in the colonies, they never became fully Spanish, and took up their own identities, and soon revolted during the 19th century.
In 1898, after revolutions in most Spanish colonies, the last colonial possession held by the Spanish were Puerto Rico, Cuba, and the Philippines. However, the United States, who aimed to expand colonially and imperialistically, saw the weak Spanish empire and its last colonial possessions as a prime and easy target. The Spanish American war saw Spain relinquish control of all 3 of its last possessions to the United States, effectively ending the Spanish Empire and the Spanish Colonial Age.